1. Field of the Invention
The present invention relates to golf balls having covers comprising a bimodal ionomeric composition. The bimodal ionomeric cover composition comprises mixtures of carboxylate functionalized terpolymers that are at least partially neutralized.
2. Description of the Related Art
Ionomeric resins (ionomers) are useful materials for the construction of golf balls, among other things. Ionomers are ionic copolymers that are obtained after copolymerization of an olefin such as ethylene with an unsaturated carboxylic acid, such as acrylic acid (AA), methacrylic acid (MAA), or maleic acid, and optionally softening monomers. Neutralizing agents—which for the purposes of this application are ionic compounds containing metal cations such as sodium or zinc ions—are used to neutralize at least some portion of the acidic groups in the copolymer resulting in a thermoplastic resin exhibiting enhanced properties. For example, golf balls constructed using ionomeric materials have improved resilience and durability as compared with balata ball construction. As a result of their resilience, toughness, durability and flight characteristics, various ionomeric resins sold by E. I. DuPont de Nemours & Company under the trademark “Surlyn®” and by the Exxon Corporation under the trademark “Escor®” and the tradename “Iotek” have become materials of choice for the construction of golf balls over the traditional balata (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability necessary for repetitive play. However, the advantages gained in increased durability of the ionomeric covers have been offset to some degree by their decreased playability. This is because the durable ionomeric resins tend to be very hard when used for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in play.
Current commercial ionomers derived from dipolymers have not been able to produce a satisfactory combination of the desirable properties of the “hard” ionomeric resins, for example, good impact resistance and carrying distance, and the desirable properties of soft balata covers, for example, playability (that is, “spin”). These are properties desired by the more skilled golfer.
Terpolymers made from copolymerization of (a) an olefin, such as ethylene (b) an unsaturated carboxylic acid and (c) other comonomers, such as alkyl acrylates and/or alkyl methacrylates, provide “softer” resins which can be neutralized to form softer ionomers. However, these softer ionomeric resins by themselves are not suitable for golf ball covers because they have reduced resilience and heat stability.
In various attempts to produce a durable, high spin ionomeric golf ball, the golfing industry has blended the hard ionomeric resins with a number of the softer ionomeric resins. U.S. Pat. Nos. 4,884,814 and 5,120,791 are directed to cover compositions containing blends of hard and soft ionomeric resins. The hard copolymers typically are made from an olefin and an unsaturated carboxylic acid. It has been found that golf balls formed from hard-soft ionomer blends tend to become scuffed more readily than covers made of hard ionomer alone. U.S. Pat. No. 5,902,855 is directed to golf balls with scuff resistant covers comprising blends of ionomers with Shore D hardness of about 40-64.
The golfing industry has also developed golf ball covers formed from polyurethane compositions. These covers combine good scuff resistance and a softness that enables spin control and good playability. Because of this combination of desirable factors, golf balls with polyurethane covers are considered to be “premium” balls for the more skilled player. However, polyurethane covers are low in resilience and hence detract from the performance of the golf ball. In addition, thermoset polyurethane covers are more difficult to process than thermoplastic ionomer resins and the material costs are also higher, making polyurethane balls more expensive to manufacture.
Thus, it would be useful to develop a golf ball cover material having a combination of softness, resilience and good scuff resistance with improved heat stability, melt processibility and lower cost. It is also desirable to develop a golf ball having a favorable combination of playability and durability.